Ultrasound transducers are frequently utilized in a variety of medical applications. The transducer may include a single element or an array of transducer elements. The transducer is typically part of an ultrasound imaging system for generating an image of a region of interest within a patient's body. For many applications, the transducer is mounted in a hand-held probe which is positioned adjacent to a selected external area of the patient's body, for example, adjacent to the chest wall to scan the heart. In other instances, the transducer is mounted in a probe that can be positioned in an internal body cavity or passage. The transducer frequently includes an ultrasound lens for focusing the ultrasound energy.
When an ultrasound transducer is used for medical imaging, it is extremely important to ensure that any materials between the transducer and the region of the patient's body being imaged do not distort or otherwise interfere with the image. In particular, when the ultrasound energy encounters an interface between materials having different sound speeds and acoustic impedances the energy can be partially reflected and refracted. Since the speed of sound in air (about 332 meters/second) is much different from the speed of sound in the human body (about 1540 meters/second) and their impedances are significantly different, it is important to eliminate air between the ultrasound transducer and the patient's body. For this reason, it is common practice to employ an acoustic gel between the transducer and the patient's body.
Single piece ultrasound lenses for flat piezoelectric crystals typically have an outer structure that is convex. This permits the transducer to better contact the body portion being imaged, and provides a lens for focusing ultrasound energy. In order to provide a lens for focusing ultrasound energy that is planar on the inside surface that contacts the transducer and convex on the outside, the sound speed of the material must be lower than the sound speed within the body. A typical material that has such a low sound speed is silicone rubber, which is relatively soft, is not durable, is quite attenuative and must be cast in place over the ultrasound transducer. It would be desirable to provide a protective cover over the silicone rubber lens. However, the cover must not significantly distort the ultrasound image or attenuate the ultrasound energy.
Another ultrasound transducer configuration involves the use of a rotating transducer and lens in a transesophageal probe as described in U.S. Pat. No. 5,127,410 issued Jul. 7, 1992. The transducer and lens are positioned behind a sealed window and rotate relative to the window. The lens includes a silicone rubber inner element and a urethane rubber outer element. A lubricant fills a gap between the surfaces of the lens and the window. The urethane rubber lens element is relatively soft and may not provide adequate mechanical support for the window in the event that an object presses or impacts against the window.